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Rebuilt LS1 piston slap or knock

Old 01-11-2019, 02:33 PM
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Default Rebuilt LS1 piston slap or knock

Just got my rebuilt 1998 LS1 back in my car and on the road and it's making some noise mostly when it's cold (link below). Local machine shop put together the short block for me. Nothing major done to the car: had some cylinder wear so they did a hone and installed forged Wiseco +4cc 3.903 pistons, stock rods, new bearings all around. Running stock 241 heads and stock cam LS7 lifters which I installed and put back together myself. Added LS6 intake and ARH 1.75" headers plus other basic bolt on's.

When car is cold and in gear (A4), I'm hearing a lot of knock or piston slap noise. I've had this car for 20 years and I am used to the piston slap sound these engines make but it usually went away after 5-10 seconds of driving. Now it takes 10 minutes of driving and it doesn't entirely go away. I'm a little concerned the machine shop or myself messed something up. Noise is noticeably louder when in gear but can still hear it faintly when in park or neutral. I've read that forged pistons and piston slap go hand in hand but this things sounds like a diesel when I first fire it up. Any thoughts on what it could be? Towards the end of the video below, the car is dropped into neutral and you can see the noise reduces drastically.

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Old 01-11-2019, 06:06 PM
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Piston slap is a real deal and the worst piston slaps I've heard are forged piston motors. Not all though. Depends how loose the pistons are, skirt length etc.

That sounds like a knock to me, like a bottom end knock. Have you driven it? Does it go away once some heat gets in the motor? If it's piston slap it should get quieter as temps go up, if it's rod knock it should get louder as temps go up and the oil thins out.

I've had some torque converter bolts back out before and cause a bottom end knock sound too. It was similar but not the same as a rod knock.

Also try pulling one coil connector at a time to see if the noise stops at a certain cylinder. If it does it's likely a rod bearing.
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Old 01-11-2019, 08:06 PM
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Originally Posted by 00pooterSS View Post
Piston slap is a real deal and the worst piston slaps I've heard are forged piston motors. Not all though. Depends how loose the pistons are, skirt length etc.

That sounds like a knock to me, like a bottom end knock. Have you driven it? Does it go away once some heat gets in the motor? If it's piston slap it should get quieter as temps go up, if it's rod knock it should get louder as temps go up and the oil thins out.

I've had some torque converter bolts back out before and cause a bottom end knock sound too. It was similar but not the same as a rod knock.

Also try pulling one coil connector at a time to see if the noise stops at a certain cylinder. If it does it's likely a rod bearing.
It definitely gets quieter after driving it for 10 minutes which is what's puzzling. When I start it up in the morning and start driving to work, it is really loud at idle in gear and in the 900-1500 rpm range - by really loud I mean I can hear it above my exhaust and radio and vent fans. If I put it in neutral, I can't hear it at all from inside the car. After about 10 minutes of stop and go traffic it's still there but much less noticeable. I've always had piston slap on this car but it would go away after I drive a few hundred feet. I've driven it about 200 miles since the rebuild and I don't think the issue is getting worse.

I've considered the flexplate/converter as well mainly because the issue is so much louder in gear when it's cold vs when it's in neutral/park. I will try unplugging the coil packs one at a time and see if that makes any noticeable change.


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Old 01-11-2019, 09:23 PM
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That's piston noise...........someone didn't size the pistons as quiet as they could be. If it doesn't shed aluminum into the oil and doesn't get worse I wouldn't worry about it. If you take a stethoscope down the side of the block at each cylinder with the engine cold you will easily find the cylinder making the noise. Another thing you could do is pull off the heads and check each cylinder to see which ones are loose in the bores. As cazy as it sounds I've actually seen machine shops measure wrong and end up with double the clearance that should be. Example lets say the spec is .003 they end at .006 thinking .003 means .003 all around. I know its sounds whacked but I've seen it a few times.

If it were mine I.d watch the oil and listen to see if it changes. Unloading the cylinders one at a time my tell you if its a bearing but from the video I hear piston noise. When a bearing is loose the piston will hit the top of the cylinder head and make more of a solid whack a whack a whack sound that gets worse with heat and usually oil pressure will flutter and lower. Piston noise is like a rocky rocky rocky sound that gets better with heat in the engine.

I had 99TA that was this noisy from the factory, motor lasted over 135k until some idiot at a trans shop beat the **** out of it and number 7 piston went out the exhaust. So it will probably last a long time.
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Old 01-11-2019, 09:41 PM
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Want to add something I remember from a while back member on here experienced. Fresh rebuilt with a similar noise, what happened was he bought new flywheel bolts and they were just a tad too long. So fully tightened the flex plate- flywheel was just a tiny bit loose causing all the noise. <long shot here

Another thing I would check all around the header flanges for dark carbon marks indicating a exhaust leak.<can sound just like a bearing or piston.

Last edited by RockinWs6; 01-11-2019 at 09:52 PM.
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Old 01-11-2019, 10:58 PM
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Ask your machinist what your piston to cylinder clearance is. Then get in touch with Weisco to cross reference the spec for your particular pistons. It sounds to me like they may have been left a little loose. Did you happen tell the builder you’re planning on boosting in the future? Thermal expansion is slower on forged pistons so I think that may be why they quiet down a bit the longer they’re running.
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Old 01-12-2019, 01:19 PM
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Have you hammered on it yet and got a lot of heat in the motor? How did it sound after that?
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Old 01-12-2019, 02:24 PM
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Some older machine shops still employ the old TRW .005” + .001” clearance mentality Newer pistons due to much less thermal expansion from better Alloys can run .003”-0035” and with Teflon skirts will end up marginally louder than stock pistons only because of the centered wrist pin location
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Old 01-13-2019, 10:29 AM
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It's good ol piston slap...nothing else. Send it!
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Old 01-14-2019, 09:36 AM
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Those have a good reputation for being quiet, so there's a good chance you have too much clearance. They need about .003" to the outside of the skirt coatings. That nets about .004 metal to metal. One thing you will hear is the knock goes away when the piston is loaded and will flutter at part-throttle low-load conditions. A lot of machine shops get in a habit of adding .001 to everything to keep out of trouble, but Wiseco has already factored in the most abusive break-in and the have never seized from being too tight. Piston Skirts can be built up with extra layers of skirt coating. It does work with reputable coating companies like Polydyn or Calico.
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Old 01-17-2019, 06:59 PM
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Thank you all for the suggestions so far. I spoke with the shop that did my block and they said they measured the pistons and gave them 0.0045" clearance. The Wiseco pistons came with a printed diagram that showed the required clearances and they referred to that when doing the work. The clearance sounds a little on the high side and maybe that's the difference between quiet operation and my tractor.

From the suggestions above: I tried pulling one the plug on one coil at a time and that didn't make any difference. I did buy new flexplate bolts but checked the length against my old ones before installing them; length was the same. I am using ARP bolts with red loctite for flexplate to torque converter. The shop used a stethoscope and felt the noise was loudest at the rear driver side and front passenger end of the engine. They recommended I use a boroscope and check to see if anything is touching the piston from the top. I am using domed pistons with stock cam, heads, and deck height. I measured piston out of bore height and found those pistons to be the highest on those pistons at around 0.018" out of the hole but figured that is fine with stock mls head gaskets. Didn't check PTV clearance, stock head and cams didn't think I needed to. Last thing I am going to check is popping the valve covers and making sure nothing is amiss there.

I've driven the car hard and the noise is there with the engine hot or cold. When it's hot I can press on the brake and bring the rpm to 1200-1500 rpm and the sound is almost as loud as it is when engine is cold and idling in gear at around 900 rpm. When hot and in park, I don't hear anything regardless of rpm.
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Old 01-17-2019, 07:43 PM
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I can tell this is drivin you nuts. Pull the heads and check the pistons, you'll know what is going on at that point OR live with it.
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Old 01-18-2019, 09:14 AM
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,033 piston to head is less than we'd recommend. What may be happening is the piston is closest to the head when cold. Then as the engine warms, the block expands upward at a faster rate than the rod/piston...which increases piston to head clearance as the engine warms. Aluminum blocks expand much further than iron. I'd pull the heads. You may see carbon being swept away from the quench pad as the piston and head contact. If you only see carbon on one side...this is good.. this means the piston hasn't completely made full contact. If you see the piston hitting on both sides, it has no where to go and the thing that is first to give is the piston skirts. They start to collapse and rock even more and then you'll have WAY more clearance in a hurry. Let us know what you find.
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Old Yesterday, 01:57 AM
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Originally Posted by RockinWs6 View Post
I can tell this is drivin you nuts. Pull the heads and check the pistons, you'll know what is going on at that point OR live with it.
It is driving me nuts. I do plan on pulling the heads but I have to arrange garage space to do the work as I don't have a garage of my own at the moment. I do appreciate your tips and insight so far. This is my first time diving into an engine so learning as much as I can as fast as I can.

Originally Posted by Summitracing View Post
,033 piston to head is less than we'd recommend. What may be happening is the piston is closest to the head when cold. Then as the engine warms, the block expands upward at a faster rate than the rod/piston...which increases piston to head clearance as the engine warms. Aluminum blocks expand much further than iron. I'd pull the heads. You may see carbon being swept away from the quench pad as the piston and head contact. If you only see carbon on one side...this is good.. this means the piston hasn't completely made full contact. If you see the piston hitting on both sides, it has no where to go and the thing that is first to give is the piston skirts. They start to collapse and rock even more and then you'll have WAY more clearance in a hurry. Let us know what you find.
I popped the plug out at the front passenger side and stuck a scope down there. I see some bright spots on the quench pad. Would this indicate there is some touching happening? I see some bright spots in other areas of the piston too so not sure what's normal and what's not. Car has about 150 miles on it since the rebuild. I've stopped driving it for now until I can get some time to work on it. The sound seemed to be getting worse when cold so didn't want to take the chance.





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Old Yesterday, 12:18 PM
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Scope a few more, cold you can get carbon build up contacting the cylinder heads. Once warmed up there should be enough clearance that it doesn't. Does look like there is contact there. Best thing to do after you pull it apart is carefully measure everything and see where you are at. If the pistons are not loose in the bores you can use a thicker head gasket to get around the noise.

As far as piston clearance is concerned, what was expected and what it is IS the question. I'd call Wiseco and pick their brains discuss what you have. I don't want to throw anyone under the bus just yet but all it takes is one loose piston to make a lot of noise. From your video it sounds like they are all on the loose side with one really out there. But this is all guessing at this point. My 175k mile ls1 is quiet cold or hot, I mean there is slight normal rocking noise cold but you can't hear it in the car, Warmed up it completely gone. BUT it doesn't have to be, depending on the piston and clearance some engines are going to be rocky sounding just shouldn't knocking sounding like this one.

Last edited by RockinWs6; Yesterday at 12:42 PM.
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Old Yesterday, 12:50 PM
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Why was this engine rebuilt? Also looks like quite a bit of scoring marks on the cylinder wall for a freshly built engine.
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Old Yesterday, 09:23 PM
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Originally Posted by Summitracing View Post
If you see the piston hitting on both sides, it has no where to go and the thing that is first to give is the piston skirts. They start to collapse and rock even more and then you'll have WAY more clearance in a hurry. Let us know what you find.
Curious as to why the skirts would collapse if the piston contacts the head on both sides.
Want to understand the mechanics involved.
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Old Yesterday, 10:28 PM
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The piston deforms around the pin area when those top areas on either side are struck. The top edges of the piston on either side of the pin area bend down, causing the skirt below them to collapse toward the center of the piston.
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